Method for treating surface of plastic

ABSTRACT

Provided is a Cr-free plastic surface treatment method which can provide a plating film sufficiently adhered to a plastic surface. The plastic surface treatment method includes treating plastic with a solution obtained by electrolyzing sulfuric acid. It is preferable that the sulfuric acid concentration of the sulfuric acid solution is 50 to 92 wt %, the persulfuric acid concentration is not less than 3 g/L, and the treatment temperature is not lower than 80° C., for example, 80 to 140° C., particularly 100 to 130° C. By immersing plastic in this sulfuric acid solution for 1 minute to 10 minutes, hydrophilic functional groups are exposed on the surface of the plastic.

TECHNICAL FIELD

The present invention relates to a method for treating a surface of a plastic (resin molded product), the method being performed prior to metallization of a surface of a plastic.

BACKGROUND ART

In portions of structural materials and component materials where metals are used, plastics are substituted for the metals utilizing advantages of the plastics, such as weight saving, reduction in cost, freedom of shape and ease of mass production. At present, plastics are widely used not only for decoration but also for exterior or interior trim parts, home appliances, and the like. In such uses, the plastic surface is subjected to plating in many cases in order to enhance rigidity, abrasion resistance, weathering resistance, heat resistance, etc.

Plastics are non-conductive, and therefore, in order to carry out plating, it is necessary to form a metal film, which becomes a conductor, on plastics first. The methods are broadly classified into dry methods such as CVD and PVD and wet methods such as electroless nickel plating. In most of the dry methods, film formation is carried out under vacuum, and they are not suitable for mass production or application to large components, so that the wet methods have been adopted so far.

Pretreatment of an ABS resin and a PEEK resin has been heretofore thought to be difficult, and in the etching process for them, a chromic acid-sulfuric acid solution has been used to thereby achieve the purpose.

Chromic acid is H₂CrO₄, and in the etching solution that is a mixed solution of chromic acid and concentrated sulfuric acid, equilibrium of

2CrO₄ ²⁻+2H₃O⁺+Cr₂O₇ ²⁻+3H₂O

is present, but Cr is hexavalent in any case. Although hexavalent Cr is an object of REACH regulation and RoHS regulation, hexavalent Cr does not remain in the products and is not subject to regulation. In recent years, however, concern about environmental issues is increasing, and environmentally friendly technology using no hexavalent Cr has been strongly desired.

As environmentally friendly technology substituted for chromic acid, it is described in a PTL 1 that etching with a mixed solution of permanganate and an inorganic salt is carried out. In the method of this PTL 1, however, surface treatment of a PEEK resin and an ABS resin is difficult, and there occurs a problem in adhesion to a metal.

CITATION LIST Patent Literature

PTL 1: JP 2008-31513 A

SUMMARY OF INVENTION Technical Problem

It is an object of the present invention to provide a Cr-free pre-plating treatment method which can provide a plating film sufficiently adhered to a surface of a plastic.

In order to solve the above problem, the present inventors have intensively studied, and as a result, they have completed the present invention comprising treating a plastic surface with a sulfuric acid solution containing persulfuric acid (oxidizing agent) obtained by electrolyzing sulfuric acid which thereby can provide a plating film sufficiently adhered to the plastic surface when the subsequent plating treatment is performed.

Solution to Problem

The present invention is based on such knowledge, and the gist of the present invention is as follows.

[1] A method for treating a surface of a plastic comprising treating a plastic with a solution obtained by electrolyzing sulfuric acid. [2] The method for treating a surface of a plastic according to [1], wherein the solution has a concentration of the sulfuric acid of 50 to 92 wt %. [3] The method for treating a surface of a plastic according to [1] or [2], wherein a treatment temperature is 80 to 140° C. [4] The method for treating a surface of a plastic according to any one of [1] to [3], wherein the solution contains persulfuric acid at a concentration of 3 to 20 g/L. [5] The method for treating a surface of a plastic according to any one of [1] to [4], the plastic being treated in a treatment equipment comprising a treatment tank for storing a sulfuric acid-containing treatment solution and an electrolytic cell for forming persulfuric acid, the sulfuric acid-containing treatment solution being circulated between the treatment tank and the electrolytic cell, wherein the plastic is immersed in the treatment tank to treat the surface of the plastic.

Advantageous Effects of Invention

According to the method for treating a surface of a plastic of the present invention, a plating film sufficiently adhered to the plastic surface can be obtained.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a structural view of a pretreatment equipment.

DESCRIPTION OF EMBODIMENTS

In the method for treating a surface of a plastic of the present invention, the plastic of interest is not particularly limited, and examples thereof include plastics having high difficulty in etching, which can not be etched unless a chromic acid-sulfuric acid solution is used, such as acrylonitrile-butadiene-styrene (ABS) resin, polyether ether ketone (PEEK) resin, polyethylene naphthalate (PEN) resin, polyethylene terephthalate (PET) resin and polyphenylene sulfide (PPS) resin.

In the present invention, the plastic is degreased first, and thereafter, the plastic is immersed in a sulfuric acid solution obtained by electrolyzing sulfuric acid, thereby treating the plastic surface. It is preferable that the sulfuric acid concentration of this sulfuric acid solution be 50 to 92 wt %, particularly 70 to 85 wt %, the persulfuric acid concentration be not less than 3 g/L, for example, 3 to 20 g/L, particularly 3 to 10 g/L, and the treatment temperature be not lower than 80° C., for example, 80 to 140° C., particularly 100 to 130° C. By the immersion in this sulfuric acid solution for 1 to 10 minutes, hydrophilic functional groups are exposed on the surface of the plastic. In the case of a PEEK resin, hydroxyl groups and carboxyl groups appear on the plastic surface.

FIG. 1 is a schematic sectional view showing an example of electrolysis equipment suitable for carrying out electrolysis of such sulfuric acid.

On the outer periphery of a treatment tank 1, a thermostatic heater 2 is provided. In the tank 1, a plastic plate 5 is arranged as a plastic to be treated in such a manner that the respective plate surfaces are directed upward and downward. In the treatment tank 1, a stirring means to stir the solution, such as a diffuser tube, may be installed.

The solution in the treatment tank 1 is circulated through a pipe 7, a pump 8, an electrolytic cell 9 and a pipe 10. In the electrolytic cell 9, an anode 9 a and a cathode 9 b each of which is formed of a diamond electrode, and a bipolar electrode 9 c arranged between them are installed. To the anode 9 a and the cathode 9 b, a predetermined current is allowed to flow from a power supply unit, and sulfuric acid is electrolyzed, thereby forming persulfuric acid such as peroxodisulfuric acid.

Using the equipment shown in FIG. 1, the surface treatment of the plastic plate 5 is carried out by placing sulfuric acid in the treatment tank 1, operating the pump 8 and the electrolytic cell 9 to form a sulfuric acid and persulfuric acid-containing treatment solution having a sulfuric acid concentration and a persulfuric acid concentration in the above ranges, and thereafter immersing the plastic plate 5 in the treatment solution in the treatment tank 1 for a predetermined time while operating the pump 8 and the electrolytic cell 9.

The plastic plate 5 taken out of the treatment tank 1 is subjected to washing with water and drying, and then subjected to plating treatment. As plating treatment methods, there are electroless plating with autocatalysis and electroless plating without autocatalysis, and any of them is available. A metal for plating may be any of nickel, copper, cobalt and alloys thereof.

EXAMPLES

Hereinafter, the present invention will be more specifically described with reference to the examples and the comparative examples. However, the present invention is in no way limited by these descriptions. In the examples and the comparative examples below, the persulfuric acid concentration measurement and the adhesion test were carried out in the following manner.

<Measuring Method for Persulfuric Acid Concentration>

First, the total concentration of oxidizing agents contained in the treatment solution was measured by iodometric titration. This iodometric titration is a method in which KI is added to liberate I₂, then I₂ is titrated with a sodium thiosulfate standard solution to determine the amount of I₂, and from the amount of I₂, an oxidizing agent concentration is determined. Next, only the hydrogen peroxide concentration was determined by potassium permanganate titration, and the potassium permanganate titration value was subtracted from the iodometric titration value, thereby determining the persulfuric acid concentration.

<Adhesion Test>

On the surface, 6 cuts penetrating down to the PEEK resin are perpendicularly made at intervals of 2 mm in each direction, and using a prescribed transparent pressure-sensitive adhesive tape, the degree of peeling is checked. The peeled portion is compared with the illustrated example and subjected to six-grade evaluation of classes 0 to 5. The class 0 means that most excellent adhesion without any peeling is exhibited. The cuts are made at 3 positions.

Example 1

Using the equipment shown in FIG. 1, surface treatment of a PEEK resin plate was carried out. The specification of the treatment tank and the conditions are as follows.

<Treatment Tank>

Volume of treatment tank 1: 40 L

Size of PEEK resin plate: 500 mm×500 mm×thickness 5 mm

<Electrolytic Cell for Forming Persulfuric Acid>

Cell volume: 0.5 L

Anode and cathode: diamond electrode (diameter 150 mm)

Bipolar electrode material: same as anode and cathode

Current density: 50 A/dm²

Solution flow rate: 52 L/h

<Surface Treatment Conditions>

Sulfuric acid concentration: 85 wt %

Persulfuric acid concentration: 10 g/L

Treatment temperature: 120° C.

Treatment time: 51 minutes

First, a sulfuric acid solution of 85 wt % was placed in the treatment tank 1, and after the persulfuric acid concentration became not less than 3 g/L by operating the pump 8 and the electrolytic cell 9, the PEEK resin plate was immersed. After the resin plate was immersed for 5 minutes, the resin plate was taken out of the treatment tank 1, washed with pure water, then dried, and subjected to electroless nickel plating by way of a catalyst application process and an activation process. The treatment conditions of each process are shown in Table 1.

From the electroless nickel-plated plate, a sample of 150 mm×100 mm was cut out, and the adhesion was examined by the aforementioned method.

TABLE 1 Treatment Treatment Process Bath temperature time name composition Concentration (° C.) (min) Catalyst hydrochloric 100 mL/L 60 10 application acid tin chloride 20 g/L palladium 0.2 g/L chloride Activation sulfuric acid 300 mL/L 50 10 Electroless Top Nicolon 200 mL/L 60 10 nickel TOM-S plating aqueous adjusted to ammonia pH = 8

Example 2 to Comparative Example 5

A test was carried out in the same manner as in Example 1, except that the surface treatment conditions were set as shown in Table 2. The results are shown in Table 2. In Table 2, the results of Example 1 are also shown.

TABLE 2 Sulfuric acid Treatment Persulfuric acid Treatment concentration temperature concentration time Adhesion Example (50 to 92 wt %) (100 to 130° C.) (not less than 3 g/L) (1 to 10 min) test result Ex. 1 85 120 10 1 class 0 Ex. 2 75 130 5 10 class 0 Ex. 3 92 100 3 2 class 0 Comp. Ex. 1 85 140 5 3 class 1 Comp. Ex. 2 45 100 10 10 class 3 Comp. Ex. 3 85 120 1 10 class 4 Comp. Ex. 4 96 100 0 10 class 4 Comp. Ex. 5 96 100 10 1 class 3

As shown in Table 2, it is seen that excellent adhesion is obtained according to the example method.

The present invention has been described in detail by using a specific embodiment, but it is obvious to those skilled in the art that various modifications can be made without departing from the spirit and scope of the present invention.

The present application is based on JP 2016-091178 filed with Japan Patent Office on Apr. 28, 2016, which is incorporated herein by reference in its entirety. 

1. A method for treating a surface of a plastic comprising treating a plastic with a solution obtained by electrolyzing sulfuric acid.
 2. The method for treating a surface of a plastic according to claim 1, wherein the solution has a concentration of the sulfuric acid of 50 to 92 wt %.
 3. The method for treating a surface of a plastic according to claim 1, wherein a treatment temperature is 80 to 140° C.
 4. The method for treating a surface of a plastic according to claim 1, wherein the solution contains persulfuric acid at a concentration of 3 to 20 g/L.
 5. The method for treating a surface of a plastic according to claim 1, the plastic being treated in a treatment equipment comprising a treatment tank for storing a sulfuric acid-containing treatment solution and an electrolytic cell for forming persulfuric acid, the sulfuric acid-containing treatment solution being circulated between the treatment tank and the electrolytic cell, wherein the plastic is immersed in the treatment tank to treat the surface of the plastic. 